Field of the Invention
The present invention relates to a novel catalytic material, to a catalyst layer comprising such a catalytic material and to its use as an electrode in an electrochemical device, in particular a fuel cell, such as a proton exchange membrane fuel cell.
Description of the Related Art
A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, such as hydrogen or an alcohol such as methanol or ethanol, is supplied to the anode and an oxidant, such as oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Electrocatalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.
Fuel cells are usually classified according to the nature of the electrolyte employed. Often, the electrolyte is a solid polymeric membrane, in which the membrane is electronically insulating but ionically conducting. In the proton exchange membrane fuel cell (PEMFC), the membrane is proton conducting, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to form water. Alternatively, the electrolyte is a liquid or molten ion-conducting electrolyte, such as phosphoric acid, as used in a phosphoric acid fuel cell (PAFC).
A principal component of the PEMFC is the membrane electrode assembly (MEA) which is essentially composed of five layers. The central layer is the polymer ion-conducting membrane. On either side of the ion-conducting membrane there is an electrocatalyst layer, containing an electrocatalyst designed for the specific electrochemical reaction. Finally, adjacent to each electrocatalyst layer there is a gas diffusion layer. The gas diffusion layer must allow the reactants to reach the electrocatalyst layer and must conduct the electric current that is generated by the electrochemical reactions. Therefore the gas diffusion layer must be porous and electrically conducting.
Electrocatalysts for fuel oxidation and oxygen reduction are typically based on platinum or platinum alloyed with one or more other metals. The platinum or platinum alloy catalyst can be in the form of unsupported nanometer sized particles (such as metal blacks or other unsupported particulate metal powders) or can be deposited as even higher surface area particles onto a conductive carbon substrate, or other conductive material (to form a supported catalyst).
The MEA can be constructed by several methods. The electrocatalyst layer may be applied to the gas diffusion layer to form a gas diffusion electrode. Two gas diffusion electrodes can be placed either side of an ion-conducting membrane and laminated together to form the five-layer MEA. Alternatively, the electrocatalyst layer may be applied to both faces of the ion-conducting membrane to form a catalyst coated ion-conducting membrane. Subsequently, gas diffusion layers are applied to both faces of the catalyst coated ion-conducting membrane. Finally, an MEA can be formed from an ion-conducting membrane coated on one side with an electrocatalyst layer, a gas diffusion layer adjacent to that electrocatalyst layer, and a gas diffusion electrode on the other side of the ion-conducting membrane.
Typically tens or hundreds of MEAs are required to provide enough power for most applications, so multiple MEAs are assembled to make up a fuel cell stack. Field flow plates are used to separate the MEAs. The plates perform several functions: supplying the reactants to the MEAs, removing products, providing electrical connections and providing physical support.
Existing state of the art electrocatalysts used in fuel cells are typically made from platinum or platinum alloys supported on conducting high surface area carbon supports. The carbon allows a high surface area of platinum to be created typically in the form of discrete nano-particles of approximately spherical geometry. The carbon support also forms a porous layer such that gaseous reactants and liquid products, or liquid reactants and gaseous products, can move to and from the electrocatalytic nano-particles. In real operational conditions, particularly at the cathode of a fuel cell where the oxygen reduction reaction occurs, a number of problems arise with carbon supported catalysts including corrosion of the carbon support and dissolution and sintering of the platinum or platinum alloy catalyst particles leading to loss of performance. A further, and more important, problem with such catalysts is that the amount of platinum needed for sufficient power output from a stack is considered too high for economic application in cost critical areas such as automotive drive trains. Therefore, of particular need, are improved catalysts for the oxygen reduction reaction that takes place at the cathode of the fuel cell. Although the existing nano-particulate platinum particles supported on carbon have high platinum surface area (surface area per unit mass of platinum—m2/g) typically greater than 50 m2/g, the surface has a low intrinsic surface specific activity (activity in terms of current generated per unit area of catalyst surface—μA/cm2) when they are equal to or less than about 3 nm in diameter (M. Shao, A. Peles, K. Shoemaker, Nano Letters, 11, 3714-3719 (2011)). This results in the overall mass activity in terms of current generated per unit mass of platinum (A/mg) being lower than required. In addition, particles of this size have poor resistance to potential cycling and can both dissolve and sinter rapidly. Also typically with the state-of-the-art catalysts, a high proportion of the carbon support surface area remains uncovered which can be oxidised, causing degradation of the catalyst, at the high potentials that occur on the cathode during routine operation and at the even higher potentials that can occur during start up and shut down periods.
One of the contributions to the low surface specific activity and to the poor stability of small platinum particles is the high number of atoms in low co-ordination sites. An atom at a low co-ordination site has fewer bonds to other platinum (metal) atoms. For example, for an octahedron, the atoms at the corners have only four near neighbours (co-ordination number is 4), atoms at an edge have seven and atoms within the (111) plane have nine near neighbours. For a small octahedron, there is a higher proportion of corner and edge sites compared to a large octahedron. Atoms at low co-ordination sites are more vulnerable to dissolution because they are not as strongly bound to the solid particle. They are also able to bond more strongly to oxygen, which means it is harder to carry out the oxygen reduction reaction; the metal tends to form an oxide rather than water.
When the particles are larger than 3 nm in diameter they are more stable and have a lower proportion of low co-ordination atoms, thus having a higher specific surface activity. However, since the surface area is lower, they typically have a lower mass activity. Since only the surface of the particle can act electrocatalytically the majority of the valuable platinum metal within the particle is not used.
Alternative structures, such as the acicular structures of platinum supported on polymer fibrils as disclosed in EP 1 021 246 B1 have been proposed as suitable alternative catalysts for the oxygen reduction reaction. These catalysts can have higher specific activities than state-of-the-art nano-particulate catalysts, but because the surface area to volume ratio of the platinum structures is still low and the platinum surface areas are therefore also low (typically around 10 m2/g platinum) (Handbook of Fuel Cells—Fundamentals, Technology and Applications, Volume 3: Fuel Cell Technology and Applications, pages 576-589), the mass activity is not sufficiently improved.
Therefore, there remains the need for improved catalysts which demonstrate higher mass activity than the current designs and which are also stable to both dissolution and sintering of the platinum, and support corrosion. It is therefore the object of the present invention to provide an improved catalytic material for use in electrochemical devices, in particular a fuel cell. The catalytic material has improved mass activity over state-of-the-art catalysts. In addition, the catalytic material has improved stability to dissolution and sintering and support corrosion.